The present invention relates generally to processing speech signals in a communication system. More specifically, the present invention relates to automatic gain control (AGC) of speech signals in the communication system.
In communication systems, a speech signal from a transmitting microphone is highly sensitive to the relative position of a user with respect to the microphone. The AGC circuit maintains the speech signal at a desired audible level by correcting the gain of the speech signal. The gain corrected speech signal is then converted into the digital format by an Analog-to-Digital converter. This digital speech signal is then encoded based on the bandwidth allocation of the transmission medium. The Analog-to-Digital converter can be either an integrated part of the encoder or a separate unit before the encoder.
A conventional method is disclosed in the U.S. Pat. No. 6,604,071 titled ‘Speech Enhancement With Gain Limitations Based On Speech Activity’. According to the method, a speech signal is divided into data frames that represent background noise as well as articulated speech activity. Gain for data frames is determined individually, both in case of background noise as well as speech activity. A limitation is applied to the determined gain of the data frames by making the gain equal to the Signal-to-Noise Ratio (SNR) and the data frames are integrated back to obtain a gain controlled speech signal. The AGC circuit uses a first order recursive filter to determine the SNR. The gain controlled speech signal is then provided at the encoder's input stage.
Another conventional method is disclosed in the U.S. Pat. No. 6,314,396 titled ‘Automatic Gain Control In A Speech Recognition System’. The method aims at differentiating a speech activity with static noise present in a speech signal. According to the method, the speech signal is divided into data frames with each data frame of a fixed time interval. An energy tracker calculates the levels of energy as high energy, low energy, and the mid energy track of the speech signal, based on high-biased running mean, low-biased running mean, and a nominally-unbiased running mean. The value of normalized energy is calculated from the high energy tracks and provided to a speech recognition system. The output of the speech recognition system is fed back to achieve optimum speech recognition.
Yet another conventional method is disclosed in the U.S. Pat. No. 5,146,504 titled ‘Speech Selective Automatic Gain Control’. This method aims to achieve AGC by converting an analog speech signal to a digital speech signal. The digital speech signal is further converted from a linear form to a logarithmic form and peak energy of the logarithmic digital speech signal is detected. The invention implements a speech recognizer to detect the speech signal. Variations in the peak energy of the speech signal are removed by a smoothing circuit. The smooth speech signal is subtracted from a reference signal and an error signal is obtained in the form of a logarithmic gain signal. The logarithmic gain signal is converted back into a linear gain signal and the linear gain signal is multiplied to the speech signal. As a consequence, AGC is used only in those cases where a speech activity is present in the speech signal. The method aims at controlling the gain of the speech signal prior to encoding.
In view of the above discussion, these conventional methods provide AGC by identifying speech activities in a speech signal and computing the energy of the speech signal. Further, the peak energy in the speech signal is detected. The detected peak energy is incremented or decremented depending on the desired audible output of speech signal.
The AGC methods discussed above use the AGC circuit as an independent module for gain correction. The gain corrected speech signal is then fed to an encoder circuit for encoding. The encoder circuit detects the energy and the speech activity in the gain corrected speech signal and thereafter converts the gain corrected speech signal from analog to digital format before encoding. This increases the time and the required rate of average Million Instructions Per Second (MIPS) for controlling the gain and encoding the gain corrected speech signal.
Therefore, there exists a need for an AGC system that aims at reducing the time, and consequently the MIPS rate, required for controlling the gain of the speech signal and encoding the gain corrected speech signal.